The answer to 'why is my bathtub overflowing' is that there is likely a clog or blockage in the drain or overflow pipe. This can be caused by hair, soap scum, or other debris accumulating over time. Symptoms may include the tub draining slowly, water backing up when the tub is filled, or the tub overflowing even with minimal water use.
In many cases, this type of issue can be resolved with some basic DIY troubleshooting and plumbing work. However, if the problem persists or you are unsure of the cause, it's best to call in a professional plumber to diagnose and fix the issue properly.